U.S. patent number 4,952,029 [Application Number 07/278,727] was granted by the patent office on 1990-08-28 for two celled liquid crystal display device with dependency of birefringence on wavelength larger in first cell.
This patent grant is currently assigned to Sharp Kabushiki Kaisha. Invention is credited to Mariko Hayashi, Yukiko Ichimura, Naofumi Kimura.
United States Patent |
4,952,029 |
Hayashi , et al. |
August 28, 1990 |
Two celled liquid crystal display device with dependency of
birefringence on wavelength larger in first cell
Abstract
There is provided a liquid crystal display device which
includes: a double-layered-type liquid crystal cell. The cell is
composed of a first cell layer and a second cell layer. The liquid
crystal cell contains liquid crystal molecules with a twisted
nematic orientation therein. Further, a voltage applying device is
in the first cell layer. The dependency of the birefringence on the
wavelength of light of the first cell layer is larger than that of
the second cell layer. Accordingly, the liquid crystal display
device produces a distinct and clear color image, and is useful for
full-color display and multicolor display.
Inventors: |
Hayashi; Mariko (Osaka,
JP), Kimura; Naofumi (Nara, JP), Ichimura;
Yukiko (Nara, JP) |
Assignee: |
Sharp Kabushiki Kaisha (Osaka,
JP)
|
Family
ID: |
17954037 |
Appl.
No.: |
07/278,727 |
Filed: |
December 2, 1988 |
Foreign Application Priority Data
|
|
|
|
|
Dec 3, 1987 [JP] |
|
|
62-306184 |
|
Current U.S.
Class: |
349/76; 349/180;
349/181 |
Current CPC
Class: |
G02F
1/133636 (20130101); G02F 1/13471 (20130101); G02F
1/1396 (20130101); G02F 1/1397 (20130101) |
Current International
Class: |
G02F
1/13 (20060101); G02F 1/1347 (20060101); G02F
1/139 (20060101); G02F 001/133 () |
Field of
Search: |
;350/335,347E,347R |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
8203468 |
|
Oct 1982 |
|
WO |
|
2092769 |
|
Aug 1982 |
|
GB |
|
Primary Examiner: Miller; Stanley D.
Assistant Examiner: Gross; Anita Pellman
Claims
What is claimed is:
1. A liquid crystal display device comprising:
a double-layered-type liquid crystal cell which is composed of a
first cell layer and a second cell layer, said liquid crystal cell
containing liquid crystal molecules with a twisted nematic
orientation therein; and
voltage applying means for applying voltage to said first cell
layer;
wherein the twist angle of the liquid crystal in said first cell
layer is opposite to that of the liquid crystal in said second cell
layer, the orientation of the liquid crystal molecules in said
first cell layer in the vicinity of said second cell layer is
orthongonal to that of the liquid crystal molecules in said second
cell layer in the vicinity of said first cell layer, and the
dependency of the birefringence on the wavelength of light of said
first cell layer is larger than that of said second cell layer such
that when voltage is applied to said first cell layer, the
dependency of the birefringence of the first cell layer is lowered
to become approximately equal to that of the second cell layer.
2. The liquid crystal display device as claimed in claim 1, wherein
the twist angles of the liquid crystal molecules in the first and
second cell layers are approximately equal to each other, and the
products .DELTA.n.multidot.d of the birefringence .DELTA.n and the
thickness d of the liquid crystal layers in each of the first and
second cell layers are nearly equal to each other.
3. The liquid crystal display device as claimed in claim 1, wherein
the twist angles of the liquid crystal molecules in the first and
second cell layers are approximately equal to each other, and the
product .DELTA.n.sub.1 .multidot.d.sub.1 of the birefringence
.DELTA.n.sub.1 and the thickness d.sub.1 of the liquid crystal
layer in one of the first and second cell layers that undergoes an
optical change due to the application of an external force, and the
product .DELTA.n.sub.2 .multidot.d.sub.2 of the birefringence
.DELTA.n.sub.2 and the thickness d.sub.2 of the liquid crystal
layer in the other cell layer that does not undergo an optical
changes are represented by the following inequality:
4. The liquid crystal display device as claimed in claim 1, wherein
the relationship between the pitch p of twist of the liquid crystal
molecules in the cell layer that has the voltage applying means
therein and the thickness d of the liquid crystal layer in the said
cell layer is as follows:
wherein 0 is the twist angle of the liquid crystal.
5. The liquid crystal display device as defined in any one of
claims 1, 2, 3 and 4, wherein a color filter layer is disposed in
at least one of the first and second cell layers.
6. The liquid crystal display device as claimed in any one of
claims 1, 2, 3 and 4, wherein the twist angle of the liquid crystal
in each of the first and second cell layers is in the range of
180.degree. to 360.degree..
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a multi-layered-type liquid crystal
display device with the use of a twisted nematic display process
that can attain an excellent color display.
2. Description of Prior Art
Liquid crystal display devices are now being used in clocks and
electronic calculating machines, in displays of computers terminal
and word processor, in televisions, and in a variety of other uses
in many fields. Recently, there has been an extremely large demand
for liquid crystal display devices because of the changes to
multicolor and full-color displays, which are already being made
use of in the fields of graphic display and image display. Color
display, that has been widely put into practical use, is attained
by a liquid crystal cell with color filter layers. The liquid
crystal cell functions as a light-switcher, and produces various
colors. The main kind of display mode is a twisted nematic display
mode attained by a liquid crystal cell in which the liquid crystal
is twisted at 90.degree.. (The "twisted nematic" is referred to
hereinafter as "TN".) However, when the TN display device is driven
at a high duty ratio, the contrast of the image decreases. On the
other hand, a supertwisted birefringence effect (which is referred
to hereinafter as "SBE") process has been suggested, which produces
TN oriented liquid crystal which is twisted at angles of
approximately 180.degree. to 270.degree. larger than 90.degree..
With the SBE process, the curve in the vicinity of the threshold
value increases steeply, and even when the duty ratio increases, it
is possible to obtain a high contrast ratio. However, because
birefringence effects of liquid crystals are used, the dependence
of the display characteristics on the wavelength of light is
theoretically higher than with the TN display in which the liquid
crystal is twisted at 90.degree.. Thus, it is very difficult to
adapt it for use in a full-color display.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a liquid crystal
display device which produces a colored display image with superior
color reproducibility and high contrast ratio even when the liquid
crystal display device is driven at a high duty ratio.
According to the present invention, there is provided a liquid
crystal display device which includes a double-layered-type liquid
crystal cell composed of a first cell layer and a second cell
layer. The liquid crystal cell contains liquid crystal molecules
with a twisted nematic orientation therein. Further, a voltage
applying device is contained in the first cell layer. The twist
angle of the liquid crystal in one first cell layer is opposite to
that of the liquid crystal in the second cell layer. The
orientation of the liquid crystal molecules in the first cell layer
adjacent to the second cell layer is orthogonal to that of the
liquid crystal molecules in the second cell layer adjacent to the
first cell layer, and the dependency of the birefringence on the
wavelength of light of the first cell layer is larger than that of
said second cell layer.
The liquid crystal display device according to the present
invention, with the use of the TN liquid crystal layer in which the
liquid crystal is twisted at 90.degree. or at an angle equal to or
larger than 180.degree., produces a distinct and clear color image.
Therefore, it is useful for full-color display and multicolor
display.
BRIEF DESCRIPTION OF THE DRAWINGS
This invention may be better understood and its numerous objects
and advantages will become apparent to those skilled in the art by
reference to the accompanying drawings as follows:
FIG. 1 is a sectional view showing the fundamental cell structure
of a double-layered-type liquid crystal display device of the
preferred embodiment according to the present invention,
FIGS. 2a and 2b are diagrams showing the twists of liquid crystal
molecules in the right and left directions, respectively,
FIG. 3 is a graph of characteristic curves showing the relationship
between the .DELTA.n.sub.2 .multidot.d.sub.2 of the second cell and
the transmittance when a polarizer is of a crossed-Nicols type with
regard to the liquid crystal display device of the preferred
embodiment according to the present invention,
FIG. 4 is a graph of a characteristic curve showing the
relationship between the twist angle of liquid crystal and the
contrast ratio of display images,
FIG. 5 is a graph of a characteristic curve showing the
relationship between the applied voltage and the transmittance with
regard to a double-layered-type SBE liquid crystal device,
FIG. 6 is a sectional view showing the fundamental cell structure
of a double-layered-type SBE liquid crystal display device of the
preferred embodiment according to the present invention,
FIGS. 7a and 7b are graphs of characteristic curves showing the
dependency of the birefringence .DELTA.n on the wavelength of light
of the liquid crystals used in the liquid crystal device shown in
FIG. 6,
FIG. 8a is a graph of characteristic curves showing the
relationship between the applied voltage and the light
transmittance with regard to a double-layered-type liquid crystal
cell comprising the liquid crystals EX-02 disposed in the first and
second cell layers, and
FIG. 8b is a graph of characteristic curves showing the
relationship between the applied voltage and the light
transmittance with regard to a double-layered-type liquid crystal
cell comprising the liquid crystal EX-02 disposed in the first cell
layer and the liquid crystal ZLI-3021-000 disposed in the second
cell layer.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows the fundamental structure of the double-layered-type
liquid crystal display device of a preferred embodiment according
to the present invention.
As shown in FIG. 1, the liquid crystal display device has a double
layered structure composed of a first and a second cell layers
C.sub.1 and C.sub.2, each of which contains a liquid crystal layer
4 therein. In the liquid crystal layer 4, liquid crystal molecules
are arranged in a twisted nematic phase. The first and second cell
layers C.sub.1 and C.sub.2 are form stacked on both surfaces of a
common transparent substrate 1. Each cell layer has such a
structure that the liquid crystal layer 4 is contained between two
transparent substrates 1 and 1' and is sealed by a sealing element
6. The orientation membranes 3 for orienting liquid crystal
molecules in a twisted nematic phase are formed on respective inner
surfaces of the substrates 1 and 1'. The first cell layer C.sub.1
has transparent electrode films 2 for applying a voltage to the
liquid crystal layer which are formed on respective inner surfaces
of the substrates 1 and 1'. Further, each cell layer has a
polarizer 5 formed on the outer surface of each of the outer
substrate 1'. Each of the transparent substances 1 and 1' may be
made of glass, acrylic resin, or a similar material. The
transparent conductive film 2 may be made of ITO (which is a film
mainly made of indium oxide), a nesa film, or a similar material.
Finally, the orientation film 3 may be made of an inorganic
material such as SiO.sub.2, SiO, or a similar material, or an
organic material such as polyimide, polyvinyl alcohol, nylon,
acrylic resin, or a similar material.
In the cell structure shown in FIG. 1, axes of respective liquid
crystal molecules in the liquid crystal layer 4 are twisted
helically from the side of one substrate to the side of another
substrate of each cell layer, as shown schematically in FIGS. 2a or
2b. As is well known to those skilled in the art, the direction of
twist is defined either that to the right (FIG. 2a) or that to the
left (FIG. 2b) when seen in the direction of incident light. In
order to give a twisting power to individual liquid crystal
molecules, at least one optically active material is added in the
nematic liquid crystal. In the case of the right twist, the
material represented by the following general formula and offered
by MERCK & Co., Inc. is added thereinto. ##STR1##
In the case of the left twist, cholesteryl nonanoate S-811.RTM.
offered by the same company is added thereinto.
Values .theta..sub.1 and .theta..sub.2 of twist angles of the first
and second liquid crystal layers 4 and 4 in the first and second
cell layers, and values .DELTA.n.sub.1 .multidot.d.sub.1 and
.DELTA.n.sub.2 .multidot.d.sub.2 of each, dictated by a product of
the birefringence .DELTA.n of the liquid crystal layer and the
thickness d thereof, are adjusted so as to satisfy the following
four requirements.
(1) FIG. 3 shows the relationship between the values of
.DELTA.n.sub.1 .multidot.d.sub.1 and .DELTA.n.sub.2
.multidot.d.sub.2 and the transmittance, under conditions that both
of .theta..sub.1 and .theta..sub.2 being set equal to 90.degree.,
respectively, with the polarizers 5 are of a crossed-Nicols type.
Moreover, a voltage is not applied to the first cell layer C.sub.1.
FIG. 3 indicates that when the value of .DELTA.n.sub.2
.multidot.d.sub.2 of the second cell layer C.sub.2 is equal to that
of .DELTA.n.sub.1 .multidot.d.sub.1 of the first cell layer
C.sub.1, the transmittance becomes the lowest. Therefore, a high
contrast ratio can be obtained. This phenomenon arises because the
light dispersion in the first cell layer C.sub.1 is compensated by
the second cell layer C.sub.2. The aforementioned results can be
obtained not only when the twist angles .theta..sub.1 and
.theta..sub.2 are set to be 90.degree., but also when they are set
to be any degree, as long as both of them are set equal to each
other. Moreover, the same results as mentioned above can be
obtained, even if the specific pitch of the twist of the liquid
crystal molecules in the first cell layer C.sub.1 is different from
that of the twist of the liquid crystal molecules in the second
cell layer C.sub.2, as long as the twisted structure of each liquid
crystal layer has a desired twist angle with respect to the liquid
crystal molecules contained therein. That is, the second cell layer
C.sub.2 functions as a compensation cell layer (a compensator) for
the first cell layer C.sub.1. This is the first requirement upon
setting those values.
In addition, taking the display contrast and visibility into
consideration, the relationship between which is shown in FIG. 4,
the twist angle of the liquid crystal is preferably set so as to
fall in the range of about 180.degree. to about 360.degree.,
wherein the contrast ratio becomes relatively high. If the twist
angle of the liquid crystal exceeds 360.degree., a domain appears
in which each liquid crystal has its orientation disarrayed at the
time of the application of voltage, resulting in a dispersion of
light. This readily causes a decrease in the contrast.
(2) To obtain a sharp steep threshold characteristic in the
contrast, the specific helical pitch p of liquid crystal is
considered to be very important. The ratio of the thickness d of
the liquid crystal layer 4 to the pitch p of the helical twist of
the liquid crystal, d/p, is preferably set so as to satisfy the
following relation which is confirmed in the experiment performed
by the inventors of the present invention:
wherein .theta. is the twist angle of the liquid crystal. This is
the second requirement. This requirement can be applied only to the
case where the pretilt angle of the liquid crystal is about
10.degree. or less. In the case that the pretilt angle of the
liquid crystals is larger than 10.degree., the range to be
satisfied by the ratio d/p becomes different from the
aforementioned range. The aforementioned requirement is available
to a normally closed displaying system in which the liquid crystal
becomes white when a voltage is applied thereto.
(3) The third requirement is also available to a normally white
display system. For example, when the values .DELTA.n.sub.1
.multidot.d.sub.1 and .DELTA.n.sub.2 .multidot.d.sub.2 in the first
and second liquid crystal layers C.sub.1 and C.sub.2 are set at 0.7
and 0.5, respectively, and when the twist angle of each of the
first and second cell layers C.sub.1 and C.sub.2 is 270.degree.,
the transmittance of light is about 70% at the time of the
application of zero voltage. The transmittance drastically
decreases, as shown in FIG. 5, with the application of voltage to
the first cell layer C.sub.1. This is because the application of
voltage causes axes of liquid crystal molecules in the first cell
layer C.sub.1 to be risen upward in the direction of thickness.
This apparently contributes to make the value of .DELTA.n.sub.1
.multidot.d.sub.1 of the liquid crystal layer 4 in the first cell
layer C.sub.1 smaller, so that the value .DELTA.n.sub.1
.multidot.d.sub.1 becomes equal to .DELTA.n.sub.2 .multidot.d.sub.2
of the liquid crystal layer 4 in the second cell layer C.sub.2. To
attain such a phenomenon, the value of .DELTA.n.sub.2
.multidot.d.sub.2 must be smaller than that of .DELTA.n.sub.1
.multidot.d.sub.1. If the value .DELTA.n.sub.2 .multidot.d.sub.2
becomes too close to the value .DELTA.n.sub.1 .multidot.d.sub.1,
the transmittance at the time of the application of zero voltage
becomes extremely low, as is shown clearly in FIG. 3. According to
the experiential consideration, it is confirmed that the value
.DELTA.n.sub.2 .multidot.d.sub.2 should satisfy the following
inequality:
Moreover, it is confirmed experientially that the requirements for
the desired twist angle of the liquid crystal and the desired ratio
of d/p thereof should be roughly the same as those in the case of
the normally closed display system.
(4) The fourth requirement is used for making the dependency, of
the display characteristics, on the wavelength of light, small. To
satisfy the fourth requirement, the liquid crystal in the first
cell layer C.sub.1 is set so that the dependency of the
birefringence .DELTA.n.sub.1 thereof, on the wavelength of light,
is larger than that of the liquid crystal in the second cell layer
C.sub.2. Moreover, by considering the speed of response, the
thickness of the liquid crystal layer 4 is preferably set equal to
or less than about 10 .mu.m or less in the case where
180.degree..ltoreq..theta..ltoreq.360.degree..
EXAMPLE 1
FIG. 6 shows the double-layered-type cell structure of a liquid
crystal display device of the preferred embodiment according to the
present invention. The twist angle of the liquid crystal in each of
the first and second cell layers C.sub.1 and C.sub.2 is 240.degree.
(i.e., the device is an SBE liquid crystal device). A transparent
conductive film 8 for applying a display driving voltage to a
liquid crystal layer 11 is formed on each of the glass substrates 9
of the first cell layer C.sub.1 alone by vapor deposition of ITO.
On each of the transparent conductive films 8, a liquid crystal
molecule orientation film 12 of polyimide is formed with a
thickness of about 500 .ANG. by a spin coating technique, the
surface of which is treated by rubbing with cloth, causing the
liquid crystal molecules to be in a parallel orientation. The
periphery of each of the first and second cell layers C.sub.1 and
C.sub.2 is sealed by a sealing element 10.
FIG. 7a shows respective dependencies of the birefringence .DELTA.n
on the wavelength of light of a nematic liquid crystal EX-02 made
by Dainippon ink & chemicals, Inc. and a nematic liquid crystal
ZLI-3021-000 (trade name) made by Merck & Co., Inc. These are
used in the preferred embodiment. As shown in FIG. 7a, it is seen
that the dependence of the birefringence .DELTA.n on the wavelength
of light of the nematic liquid crystal EX-02 is larger than that of
the nematic liquid crystal ZLI-3021-000. Therefore, the nematic
liquid crystal EX-02, to which 0.8 wt % of an optically active
material S-811 is added, is used in the liquid crystal layer 11 of
the first cell layer C.sub.1. Further, the nematic liquid crystal
ZLI-3021-000, to which 1.2 wt % of CB-15 is added, is used in the
liquid crystal layer 11' of the second cell layer C.sub.2. The
pitch of the liquid crystal layer 11 in the first cell layer
C.sub.1 is about 10 .mu.m and the pitch of the liquid crystal layer
11' in the second cell layer C.sub.2 is about 15 .mu.m. The twist
angle of the liquid crystal in the first cell layer C.sub.1 is
opposite to that of liquid crystal in the second cell layer
C.sub.2. The thickness of the liquid crystal layer 11 in the first
cell layer C.sub.1 is about 6 .mu.m and that of the liquid crystal
layer 11' in the second cell layer C.sub.2 is about 9 .mu.m. A
crossed-Nicols type the polarizer is used for each of polarizers 7.
FIG. 8b shows the dependence of the light transmittance on the
applied voltage with regard to the double-layered-type TN (twisted
nematic) liquid crystal device of the present example. It includes
the liquid crystal layer 11 of the nematic liquid crystal EX-02 and
the liquid crystal layer 11' of the nematic liquid crystal
ZLI-3021-000. FIG. 8a shows the dependence of the light
transmittance on the applied voltage with regard to the
double-layered-type TN liquid crystal device of a comparative
example. It includes the liquid crystal layers 11 and 11' of the
nematic liquid crystal EX-02. The wherein the wavelengths .lambda.
used herein for red color, green color, and blue color are 610 nm,
550 nm, and 450 nm, respectively. As shown in FIG. 8b, since the
characteristic curves for red color, green color and blue color of
the TN liquid crystal device of the present example show
substantially uniform characteristics at in the range from the
threshold voltage to a higher voltage, the TN liquid crystal device
of the present example is useful for a color display with high
contrast.
EXAMPLE 2
An experiment with regard to a double-layered-type TN liquid
crystal device, including the liquid crystal layer 11 of a nematic
liquid crystal SP4024 made by Chisso and the liquid crystal layer
11' of the nematic liquid crystal ZLI-3021-000, was made as well as
the experiment of the Example. The dependence of the birefringence
.DELTA.n on the wavelength of light of the nematic liquid crystal
SP4024 is larger than that of the nematic liquid crystal
ZLI-3021-000, as shown in FIG. 7b. It is seen from the result of
the experiment of the Example 2 that the TN liquid crystal device
used in the Example 2 has the same effect as that of the TN liquid
crystal device of the preferred embodiment used in the Example
1.
In the aforementioned examples, color filter layers of red, green
and blue are disposed inside of the liquid crystal cell layer with
a voltage applying device of each of the TN liquid crystal display
devices. The liquid crystal display devices with the color filter
layers are subjected to a duty drive, with the formation of a
distinct, clear color image. These liquid crystal display devices
are useful for full-color display and multicolor display.
It is understood that various other modifications will be apparent
to and can be readily made by those skilled in the art without
departing from the scope and spirit of the present invention.
Accordingly, it is not intended that the scope of the claims
appended hereto be limited to the description as set forth herein,
but rather that the claims be construed as encompassing all the
features of patentable novelty that reside in the present
invention, including all features that would be treated as
equivalents thereof by those skilled in the art to which the
present invention pertains.
* * * * *